The present invention relates to electrical energy storage systems which may be recharged over numerous cycles to provide reliable power sources for a wide range of electrical utilization devices. The invention is directed in particular to a rechargeable storage system which is capable of exhibiting both high energy density normally associated with batteries, and high power density and long operative life typical of supercapacitors.
In the present invention, such a system comprises a multi-layer energy storage device structure which incorporates respective positive and negative electrode elements comprising pseudocapacitor or double-layer supercapacitor materials and rechargeable intercalation battery materials in a unitary, flexible structure which may be sized and shaped as desired to be most compatible with utilization devices while providing advantageously high energy and power densities.
Modern applications requiring mobile electrical energy sources, ranging from personal telecommunications devices to electric vehicles, are proliferating at an exponential rate. The demands of these applications range widely, for example, in voltage or power level, but all are preferably served by light-weight storage devices which can rapidly provide consistently high energy density over long time spans and can be quickly recharged to operational energy levels. To date, these extensive mobile energy needs are being met, in a fashion, by one or the other of the two available types of storage devices, viz., rechargeable batteries, such as lithium-ion intercalation systems, or supercapacitors of either faradic pseudocapacitive or non-faradic double-layer reaction type.
The choice between these battery or supercapacitor systems is normally dictated by the more pressing of the application's demand for high energy density, available from batteries, or for the rapid delivery of high power, provided by supercapacitors. Attempts to meet requirements for both high energy and high power densities in a single application have led in some instances to the utilization of both device types arranged together in such a manner that the battery is available to recharge the supercapacitor between periods of high power demand. The disadvantage of such a practice in the excessive weight factor alone is clearly apparent. Additional limitations on this expedient are reflected in the time requirement for battery charging, as well as in the multiplicity of cells and in battery life cycle which may often be shortened by the physical rigors of the intercalation battery charging operation.
The system of the present invention represents a remarkable advancement in means for meeting the requirements of mobile electrical energy utilization in that it combines the desirable characteristics of both the battery and the supercapacitor in a single integrated device of light weight and extended energy capacity. Comprising opposing electrodes of, for example, an activated carbon supercapacitor element and an intercalatable battery composition, particularly a transition metal oxide spinel material having a structure which exhibits rapid ion diffusion and little physical distortion from intercalation, the system is able to exhibit both the high energy storage capability of batteries and the high speed power delivery and exceptional cycle life of supercapacitors. An additional advantage of this unique combination of faradic battery intercalation and capacitive surface charging is the realization of intercalation systems which would not otherwise be available due to the sparsity of receptive counter-electrode materials able, for instance, to accommodate cations of considerable size, e.g., alkaline earth cations.
The hybrid systems of the present invention can utilize most of the respective compositions of previous rechargeable intercalation batteries and supercapacitor devices. Such earlier devices are typically represented, e.g., in U.S. Pat. Nos. 5,418,091 and 5,115,378. As in these earlier systems, intercalating electrodes may comprise metallic sulfides, oxides, phosphates, and fluorides, open-structured carbonaceous graphites, hard carbons, and cokes, and alloying elements, such as aluminum, tin, and silicon. Similarly, surface-active capacitor materials, typically high surface area closed-structure activated carbon powders, foams, fibers, and fabrics may be used in the counter-electrodes. The additional active electrolyte element of the hybrid systems may likewise be formulated of prior available materials, with particular utility being enjoyed in the non-aqueous solutions of intercalatable alkali and alkaline earth cations, usually incorporated in significantly fluid form in fibrous or polymer matrix containment materials, thus maintaining an environment conducive to mobility of both species of electrolyte ions. The laminated polymeric layer format typified by the secondary batteries described in U.S. Pat. No. 5,460,904 and related publications serves well for the structures of the present invention.